Boreholes put down in the course of opening a mine near Thistleton, eastern England, show that boulder clay conceals a narrow, deep, steep-sided valley in the Lincolnshire limestone, filled with glacial drift. The Lower Estuarine Series, Northampton ironstone, and upper Liassic clay underlie the limestone. Plotting of structure contours has revealed "bulging" of the Liassic clay, a characteristic previously recognized elsewhere in the eastern Midlands.